One knows and uses various devices by means of which orifices of different diameter or size are positioned successively to traverse a conduit, each orifice permitting the measure of flow with sufficient precision in a range or interval of flow whose median corresponds to the optimal precision.
Since the intervals of measure are arranged in a continuous series, one obtains a range of values of flow which can be large but is of fluctuating precision, optimal for the median value of flow of each orifice, but reduced at the limits. In all of these arrangements, the passage or change from one interval to the next is discontinuous since it is necessary to change the plate of the orifice.
Thus, U.S. Pat. Nos. 1,903,068 filed Mar. 4, 1931 and 1,965,826 filed May 6, 1931 describe devices comprising a chamber permitting the removal of an orifice plate with one size opening and replacement with another having an orifice opening of a different size.
Automation of the substitution of one orifice for another of different size has been attained either by translation of a single plate having plural orifices of different sizes, as in U.S. Pat. No. 3,105,384 filed Jan. 23, 1961, or by rotation of a plate having plural orifices whose centers are on a common circle, as in U.S. Pat. No. 3,079,796 filed Feb. 4, 1960.
These devices necessarily have a large chamber whose seals are the object of frequent inspection and which make use difficult remote from the centers of production sites, particularly on sub marine soil and also where very high pressures must be handled.